In liquid crystal devices it is known that alignment layers are employed on the inner surfaces of opposed substrates to orient and provide a well-defined disposition of the optic axis of liquid crystal material between the substrates. The alignment layer is typically made up of a long chain polymeric material which is later subjected to processes such as mechanical rubbing or ultraviolet exposure to alter the surface properties.
There are four generally accepted techniques for forming an alignment layer on the substrate of a liquid crystal device. Commonly used methods are rubbing or photo-alignment of organic/polymer films, and evaporation of inorganic materials. Although each method is capable of aligning the liquid crystal material, each method has particular drawbacks.
The most commercially used method of forming an alignment layer is the rubbing method. In this method, for example, a polyamic acid is spin-coated or otherwise deposited on a substrate and then imidized by subjecting it to two heat treatments (soft and hard bake) cycles forming a polyimide (PI) film. After an appropriate cooling period, the PI film is rubbed by a cloth, such as velvet, in a uniform singular direction. This orients the liquid crystal material that later comes in contact with the rubbed surface, along the rubbing direction. Unfortunately, this method can cause mechanical damage and generate electrostatic charges, both of which adversely affect liquid crystal displays that employ thin-film transistors. This method also generates dust from the cloth and PI which may adversely contaminate the liquid crystal material.
Another method of alignment is where the polyamic acid is disposed on the substrate and polymerized (imidized), as in the above method, resulting in a PI film. Next, after imidization, a linearly polarized ultraviolet light is projected onto the surface of the substrate to form the desired molecular alignment. The UV radiation anisotropically photo-dissociates photosensitive bonds in the PI including those in the imide ring. This selectively reduces the polarizability of PI molecules and changes the surface properties and morphology. Unfortunately, this method has resulted in alignment layers with weak anchoring of liquid crystals and poor thermal and chemical stability. This method also requires costly multi-step processing. Yet another drawback of this method is that it only provides a limited charge holding ratio and less thermal stability when compared to the rubbing method.
A similar method of preparing an alignment layer which uses photo-sensitive polymers is also known. For example, photo-sensitive polymers such as poly(vinyl)4-methoxycinnamate (PVMC); poly(vinyl) cinnamate (PVC); and polysiloxanecinnamate films may be used to align liquid crystal material. These materials, when exposed to a linearly polarized ultraviolet light (LPUV), initiate a photo-reaction after evaporation of the solvent. This method causes bonding and resultant orientation of the side chain molecules'axes uniaxially in a direction determined by the direction of linear polarization. However, this process does not chemically fix the orientation of the molecules and the alignment is reversible with exposure to normally occurring ultraviolet light. Moreover, the chemical composition of the materials is lost over time. As such, this process does not provide an alignment layer with a fixed, stable orientation of liquid crystal material later disposed thereon.
Yet another method for forming an alignment layer on a substrate is to deposit by evaporation inorganic materials such as SiO.sub.x, MgO, etc., onto the surface of the substrate at various incidence angles. This forms an alignment layer which physically orients the director of the liquid crystal in the desired direction with or without a pre-tilt. However, this cumbersome method has been found difficult to use in a manufacturing process.
In light of the foregoing, it is evident there is a need in the art for a non-contacting method of forming an alignment layer for substrates used in liquid crystal devices. Moreover, there is need for a method which provides the desired alignment direction with chemical and thermal stability and improved anchoring strength and which utilizes a simpler, low cost, dust-free, and electrostatic charge free manufacturing process.